1. Field of the Invention
The invention relates to an exhaust system of an internal combustion engine.
2. Description of the Related Art
In an exhaust system of an internal combustion engine, such as a V-type engine, having a plurality of cylinders, all of the cylinders are divided into two cylinder groups, each of which is provided with an exhaust manifold, and an exhaust passage is provided for each of the cylinder groups such that the exhaust passage extends from the corresponding exhaust manifold to an exhaust merging point at which streams of exhaust gas from the respective exhaust passages join together. In the exhaust system of this type, it has been proposed that a turbine of a turbocharger and a catalytic converter downstream of the turbine are disposed only in one of the exhaust passages, and this exhaust passage communicates at a junction point upstream of the turbine with a junction point of the other exhaust passage, while a control valve is disposed downstream of the junction point of the other exhaust passage. An example of the thus constructed exhaust system is disclosed in, for example, JP-B2-1-27246.
In the arrangement as described above, the control valve is used as a wastegate valve of the turbocharger, and the control valve is closed during low-load engine operation in which a small amount of exhaust gas is produced, so that the exhaust gas of all of the cylinders passes the turbine. During high-load engine operation in which a large amount of exhaust gas is produced, the control valve is opened so that the exhaust gas of the other cylinder group passes the other exhaust passage, and only the exhaust gas of the above-indicated one cylinder group passes the turbine. In this manner, the boost pressure established by the turbocharger is constantly controlled to a set value or its vicinity.
When the vehicle is decelerating, a fuel-supply cutoff operation is usually performed to reduce fuel consumption. In the exhaust system described above, if the control valve is closed, due to a low engine load, even during the fuel-supply cutoff period so that the exhaust gas of all of the cylinders passes the turbine, the exhaust gas of all the cylinders passes the catalytic converter. If the temperature of the catalytic converter is high, catalysts, formed of noble metal, usually carried on the catalytic converter, deteriorate due to sintering caused by the exhaust gas containing a large amount of oxygen emitted during the fuel-supply cutoff period. If the single catalytic converter deteriorate due to deterioration of the catalysts, the exhaust gas cannot be purified.
The invention provides an exhaust system of an internal combustion engine, in which exhaust gas is appropriately purified even if a catalytic converter disposed downstream of a turbine deteriorates during a fuel-supply cutoff period.